Going to try for a new tow vehicle

I bought new a 2005, 2500 RAM, 4 door, short bed, Cummins (24 valve) with 2.76 gearing (did a lot of highway driving at the time). Highway (21-22 mpg), city (13 mpg), average (16-18 mpg). Pulls anything (its weight is 6,700 lbs). Use mostly as a daily driver now (190,000). 300,000 is a norm for this motor, but I do follow and do all the maintenance myself. I'm hoping to get 4-500,000 out of it. Yes, it is costly to maintain, 12 qts. of oil & filter every 7,000 miles, fuel filter, 15,000 miles, but being HD, other mechanical parts are built stronger and last longer. Example: 5 cooling systems. "E" rated tires on 17" rims, makes it a bull dog.